The present invention relates to hydrodynamic couplings, and more particulary to improvements in hydrodynamic fluid couplings of the type known as Fottinger couplings. Still more particularly, the invention relates to improvements in hydrodynamic couplings including couplings of the type wherein some fluid is continuously evacuated from the radially outermost region of and a pump or the like delivers fluid to the working chamber.
It is already known to provide a Fottinger coupling with an opening which is located close to and communicates with the radially innermost portion of the working chamber to permit evacuation of working fluid in response to increasing torque and resulting higher slip of the impeller relative to the turbine wheel or runner. In some instances, the fluid which is to leave the working chamber by way of such opening (the opening is provided in the nave or hub of the impeller or runner) is caused to flow along a suitably configurated deflector. Reference may be had to page 388 of "Motortechnische Zeitschrift" (Vol. 11, 1958) which discloses a coupling for use as a safety device in ice breakers, or to Swiss Pat. No. 337,371. When the RPM of the runner is suddenly reduced (e.g., as a result of the braking action of an obstacle which is encountered by the propeller screw of an ice breaker) and the slip of the impeller relative to the runner increases accordingly, the aforementioned opening allows for rapid evacuation of fluid from the working chamber. This results in immediate reduction of the load upon the drive. When the coupling is used under the just outlined circumstances (e.g., as a safety device between the prime mover and the propeller screw of an ice breaker), the opening invariably insures rapid evacuation of surplus fluid from the working chamber.
However, a coupling wherein the aforediscussed opening is in direct communication with the working chamber and is located close to the radially innermost portion of such chamber is not sufficiently versatile to warrant its use in other types of machines or apparatus, e.g., in a vehicle where the coupling transmits torque from the engine to a cooling fan. In such constructions, the coupling should insure that the RPM of the fan varies in dependency on the desired cooling action and also to limit the magnitude of transmitted torque. Rapid increase of slip of the impeller relative to the runner (which latter drives the fan) is not attributable to rapid deceleration of the runner but rather to rapid increase of RPM of the primer mover, i.e., to rapid increase of the RPM of the impeller, and to relatively large mass (inertia) of the rotary part or parts of the fan.
The situation is aggravated if the vehicle wherein the coupling transmits torque to a cooling fan is equipped with an automatic transmission which is designed to change speeds while under load. In such vehicles, the RPM of the engine increases very rapidly in response to shifting to another speed, i.e., the rotational speed of the impeller can rise abruptly when the driver decides to shift to a different speed ratio. The problem is especially acute when the driver shifts into a lower gear because this can result in such pronounced rise of RPM of the impeller that, when using a conventional hydrodynamic coupling, the transmitted torque temporarily rises to a value which is 4-8 times the rated value. This can result in partial or complete destruction of the coupling and is rather surprising since one of the most important characteristics of a hydrodynamic coupling is that it is capable of eliminating peaks of transmitted torque and thus constitutes a safety device against overloading of driven components.
Certain other proposals to limit the magnitude of transmitted torque include the provision of a so-called deflector which is installed in the radially innermost region of the working chamber and does not obstruct the circulation of working fluid when the slip is relatively small. When the slip increases, the deflector influences the circulation of working fluid and thereby limits the magnitude of the transmitted torque. Reference may be had to U.S. Pat. No. 2,301,645 granted Nov. 10, 1942 to Sinclair. It was further proposed to provide a so-called accumulator chamber radially inwardly of the blading of the impeller so that the accumulator chamber is in direct communication with the spaces between the blades of the runner. When the slip increases, the accumulator chamber receives some of the working fluid which is thus withdrawn from the working chamber to thereby limit the magnitude of the transmitted torque. Such feature is disclosed in U.S. Pat. No. 2,875,581 granted Mar. 3. 1959 to Kugel.